1. Field of the Invention
The present invention relates to a circuit arrangement for a blocking oscillator switching power unit which comprises a transformer having at least three windings at the primary side and at least one winding at the secondary side, a switching transistor which periodically applies a rectified and smoothed input voltage to a first winding at the primary side, an integrated circuit which drives the switching transistor, receives a rectified and smoothed supply voltage via a second winding at the primary side during the blocking phase and receives a test voltage via a third winding at the primary side during the flow phase, whereby an input of the integrated drive circuit registering the zero-axis crossings of the test voltage is connected to the one end of the test winding and the supply voltage input of the drive circuit is connected to the one end of the supply winding via a diode and to an a.c. voltage input of the input voltage rectifier via a resistor.
2. Description of the Prior Art
A blocking oscillator circuit of the type generally set forth above is described in the Siemens Brochure "Schaltnetzteile mit der IS TDA 4600", (FIG. 20).
In the known circuit, the drive module receives its energy from a supply winding at the primary side that is wound such that the supply voltage follows the stabilized output voltage. The range of regulation is correspondingly high and the circuit can operate with high voltages between 90 volts and 270 volts. It is disadvantageous, however, that the output voltage, after the device has been switched on, can only build up with the same speed with which the output capacitors are charged. Measures must therefore be undertaken in order to assure that the drive circuit already receives an adequately-high voltage in the start-up phase. The initially mentioned publication provides that an alternating voltage be taken from the network for this purpose, this voltage being smoothed, rectified and then applied to the supply input of the drive circuit. This start-up aid, however, does not always operate satisfactorily; it cannot be dimensioned such without further measures that, on the one hand, it supplies enough energy in the start-up phase and, on the other hand, does not have a disturbing effect during normal operation.